Modern multimedia systems often employ predictive coding techniques to accommodate transmission of video in a bandwidth-limited channel, such as wireline networks, wireless networks and even storage devices (such as optical disk systems). Predictive coding techniques reduce bandwidth of a source video sequence by exploiting spatial and/or temporal redundancies in the source video. For example, content of one frame may be coded using one or more other frames as a source of prediction. Such techniques can be efficient when there is strong temporal and/or spatial correlation among frames.
Temporal prediction, for example, endeavors to predict a frame with reference to one or more “reference frames.” Reference frames are frames that have been coded by an encoder, transmitted in a channel and, by the time a given frame is to be decoded, already decoded by a decoder. The reference frames also are decoded by the encoder so the encoder and decoder have common copies of the decoded reference frames. Thereafter, for a new frame to be coded, an encoder may search among the locally-stored reference frames to determine which frames, if any, provide good sources of prediction for content in the new frame. The prediction search often is performed at granularities smaller than a frame, for example, after the frame has been parsed into sub-units (called “pixel blocks” herein). When a prediction match is found, the encoder may transmit an identification of the matching content in the bit stream, and then code differences between content in the new frame and content in the reference frame from which the prediction reference is made.
Zooming transitions can interfere with prediction search operations. Zooming transitions can occur when image content in a field of view increases or decreases during a portion of a video sequence. Although temporal correlation exists between the frames, the changing size of image content can interfere with an encoder's ability to develop a prediction match between a frame being coded and one or more reference frames.
The inventors perceive a need in the art to develop coding techniques to code a video sequence predictively when zooming transitions occur in the sequence.